OK get some RO water take 2 cups of Banking Soda, and cook it at 300 degrees for 1 hour. Then gallon of the RO water add some slowly to your tank. key word is slowly. Is you tank getting a good supply of oxygen in the winter months oxygen levels are lower inside causing PH to drop. You can also turn your lights in over a sump in off hours.
well I will finish what 180 was saying...First off DON"T dose all the baking soda!!!!!!!!!!! also, what size tank? you also have to be careful because you are raising your alklinty with baking soda...that's why I was wondering your cal,alk, & possibly mag (only if you test for it). Is the tank in the basement? furnace close? carbon dioxide will keep your PH low....try to import some fresh air if possible